Since relatively few oil wells flow from natural pressures, most wells require a pumping unit to remove the oil. One type of pumping unit heretofore used comprises a base, a beam (known in the art as a "walking beam") pivotally mounted on the base, and an electric or internal combustion engine which, through a large speed reducer, drives a linkage connected to one end of the beam for rocking the beam on the base about a generally horizontal axis. The other end of the beam is connected, by means of a cable, for example, to a polished rod which extends vertically through the well head. A string of sucker rods extend from the polished rod down into the well inside a well casing to a plunger-type pumping valve which is positioned adjacent the oil-producing formation. As the beam is rocked on the base, the sucker rods reciprocate up and down in the well casing and oil is pumped up and out the well casing to storage vessels.
This type of prior art pumping unit is subject to a number of disadvantages, one being that its pumping motion is relatively jerky, with abrupt changes in acceleration and deceleration of the beam. This type of motion causes undue stress in the sucker rods, resulting in metal fatigue and premature rod failure. The tendency for rod failure to be incurred in the use of conventional pumping units is aggravated by the operation of the unit to force the rods through any obstructions in the well.
Another disadvantage of a beam-type pumping unit arises from the fact that the stroke of the unit is shorter than a sucker rod which is typically 25-30 feet in length. Thus, the unit cannot be used to pull a broken or otherwise defective rod from the well. Instead, the pumping unit must be removed from the well and a special tower installed to remove the sucker rod. This is a time-consuming and expensive operation, especially since the well remains unproductive during the entire procedure. The fact that a typical beam-type pumping unit has a relatively short stroke is further disadvantageous in that the unit pumps only a relatively small amount of oil per pumping cycle from the well. Therefore, a relatively large number of cycles are required to pump a given quantity of oil from the well. This, of course, also tends to reduce the working life of the sucker rods. In addition, this type of prior art pumping unit utilizes large counterweights on the walking beam to assist in the oil pumping operation and requires the use of power units having substantial horsepower ratings.
One proposed solution to the problems associated with conventional beam-type pumping units is disclosed in my U.S. Pat. No. 3,651,635 for a Pumping Jack, issued Mar. 28, 1972. The pumping jack of that patent includes an elongate tubular member pivotally mounted at its middle on a base for rocking on the base about a generally horizontal axis. The jack further comprises a pair of spherical reservoirs at opposite ends of the member and in fluid communication with the latter. Transfer of liquid from one reservoir to the other and vice versa through the tubular member is effected by a pump and valve assembly and causes the member to rock on the base about its pivot so as to achieve the desired pumping action. This pumping jack represents an improvement over conventional pumping units in that it provides a smooth, controllable pumping action and has no tendency to snap the sucker rods. On the other hand, the pumping jack of the aforementioned patent still has a relatively short stroke, requires a large number of pumping cycles to pump a given volume of oil from the well, and cannot be used for replacing rods or otherwise servicing the well.
Another prior art pumping unit designed to avoid the problems associated with beam-type pumping units is presently being marketed by the Oilwell Division of United States Steel Company. This pumping unit includes a tower, a pulley mounted at the top of the tower, and cables extending over the pulley. Each cable is attached at one end to a polished rod load (i.e., a polished rod and sucker rods) outside the tower and at its other end to a counterweight inside the tower. A large motor, acting through a speed reducer, on top of the tower drives the pulley in one direction to assist in raising the counterweight as the sucker rod string descends into the well and then in the other direction to assist in raising the sucker rod string to pump oil from the well. While this unit solves some of the problems associated with the beam-type pumping units, others remain. The primary advantage of the Oilwell pumping unit is that it can be made to have a relatively long stroke (e.g., 30 feet) for increasing the amount of oil pumped from the well during a cycle of pumping operation. Thus, fewer pumping cycles are required to pump a given quantity of oil from the well and the rods suffer a correspondingly fewer number of cyclic stresses for a longer working life. On the other hand, the pumping unit tends to impart a relatively uneven motion to the sucker rods. The units are also quite heavy and expensive and require motors having substantial horsepower ratings and power consumption for operation.